1. Field of the Invention
This invention generally relates to electrochemical batteries and, more particularly, to a battery cathode comprised of a Prussian blue analogue with no zeolitic water content.
2. Description of the Related Art
Prussian blue analogues (PBA), often referred to as transition metal hexacyanometallates (TMHMs), have been investigated as cathode materials for rechargeable lithium-ion batteries [1, 2], sodium-ion batteries [3, 4], and potassium-ion batteries [5]. With an aqueous electrolyte containing the proper amount of alkali-ions or ammonium-ions, copper and nickel hexacyanoferrates ((Cu,Ni)-HCFs) exhibited a very good cycling life where 83% capacity was retained after 40,000 cycles at a charge/discharge current of 17 C [6-8]. However, electrochemical devices using PBAs as a cathode material with an aqueous electrolyte can only operate at low voltages (less than 2 volts (V)) because the water in the electrolyte can be decomposed by the process of electrolysis, which occurs at voltages greater than 1.23 V. On the other hand, electrochemical devices with PBA cathodes and non-aqueous electrolytes have a higher voltage output and, therefore, a higher energy density. Manganese hexacyanoferrate (Mn-HCF) and iron hexacyanoferrate (Fe-HCF) were used as cathode materials in non-aqueous electrolyte [9, 10]. Assembled with a sodium-metal anode in a half cell configuration or with hard carbon anode in a full cell configuration, Mn-HCF and Fe-HCF electrodes have the capacity to cycle between 2.0V and 4.5 V and to deliver reversible capacities of greater than 140 milliamp hours per gram (mAh/g).
FIG. 1 is a diagram depicting the open framework structure associated with the general formula of AXM1MM2N(CN)Z (prior art). The open framework structure of the TMHMs facilitates both rapid and reversible intercalation processes for alkali (Group 1A), alkaline (Group 2A), and Group 3A ions (AX). The capacity of the TMHM is determined by the available A-sites in the compounds into which the alkali, alkaline, and Group 3A ions can be intercalated reversibly in the range of working voltages.
FIG. 12 is a schematic diagram depicting the redox potential of various transition metals vs. Na°. The drawings depicts why a non-aqueous electrolyte must be selected if a battery operating voltage is to exceed the 1.23 volt water electrochemical window (E.W.). Since Mn and Fe have a redox potential between 2-4 V vs. Na°, sodium-ions can be intercalated/deintercalated into/from Na2MnFe(CN)6 between 2-4 V vs. Na°, and its theoretical capacity is 171 mAh/g. Similarly, 2 sodium-ions can be intercalated/deintercalated into/from Na2FeFe(CN)6 between 2-4 V vs. Na° and its theoretical capacity is also around 170 mAh/g. However, for Na2FeCu(CN)6, only one sodium-ion per formula can be reversibly inserted/removed into/from the compound because the redox potential of Cu3+/2+ is higher than 4 V vs. Na°. Its theoretical capacity is 83 mAh/g, which is about half the value of Na2FeFe(CN)6 or Na2MnFe(CN)6. Accordingly, an electrolyte with a wider electrochemical window must be used if Cu3+/2+ is the active material in a sodium-ion battery. It is worth noting that a proper anode, with a low working potential to match the water reactive area, is also required for a battery in order to achieve a high operation voltage (i.e. greater than 1.23 volts).
Due to the large interstitial spaces, it is also inevitable that water molecules readily occupy the A-sites in PBAs during the material synthesis process. The behavior of water absorption in PBAs resembles a process where water is absorbed by zeolitic materials. As a result, at least one researcher has referred to these water molecules as zeolitic water [14]. Accordingly, the PBA formula is often written as AXM1MM2N(CN)Z.dH2O, where dH2O is zeolitic water. This same reference (Wessells) states that, at least in theory, the value of d may be zero. However, this analysis is inaccurate. First, this same references states that their electrochemical device is not stable if all the water is removed from the PBA lattice. Second, different amounts of water in PBA result in different battery configurations. Third, a synthesized PBA material includes two types of water. One type of water is the above-mentioned zeolitic water, which might also be referred to as interstitial water. The second type of water is bound water, which might also be referred to as lattice or lattice-bound water. Managing these two types of water in a PBA lattice is a key to making different batteries using PBA materials.
As noted in the Encyclopedia Britannica's discussion of clay-water relations (http://www.britannica.com/science/clay-mineral/Clay-water-relations#ref618526), “(t)he water adsorbed between layers or in structural channels may further be divided into zeolitic and bound waters. The latter is bound to exchangeable cations or directly to the clay mineral surfaces. Both forms of water may be removed by heating to temperatures on the order of 100°-200 degree C. and in most cases, are regained readily at ordinary temperatures. It is generally agreed that the bound water has a structure other than that of liquid water; its structure is most likely that of ice.” Alternatively stated, is that zeolitic water is physically trapped in the crystal structure, whereas the bound water is chemically bonded with the crystal. Because of the chemical bond, a higher temperature is needed to remove the bound water from a crystal.
While it is possible to remove zeolitic water from a PBA compound, it can only be done using a high temperature process, as disclosed herein. As explained in detail below, it is not possible to completely remove zeolitic water using the 70 to 100 degree C. temperatures disclosed in the Wessells application [14], and the resultant PBA material therefore includes at least 20% zeolitic water by weight. The zeolitic water occupies the void (˜0.35 nanometer (nm) in diameter) at the center of the PBA lattice. Wessells suggested that because the hydrated A-cations (Na+, K+, Mg2+, Ca2+, Ba2+) have a Stokes ionic diameter of about 0.35 nm, the hydrated sodium and potassium in electrolyte might be exchanged with zeolitic water already present in the crystal structure that contribute to the mechanism for ion transport through the lattice. The PBA materials disclosed herein, however, permit the fabrication of electrochemical devices with much higher energy density than Wessells' device. In order to do so, the PBA electrode needs to have higher capacity (mAh/g), and it must be to operate at higher voltages. If the zeolitic water is not completely removed from the PBA material it is not feasible to make an electrochemical device with a higher capacity and voltage using such a PBA as a cathode. One requirement for higher voltage electrochemical devices is the use of a non-aqueous electrolyte, since electrolysis (water decomposition) occurs at voltages higher than 1.23 volts. Most importantly, the zeolitic water should be avoided in the PBAs because it can move to the non-aqueous electrolyte freely to reduce its electrochemical window.
The other type of water found in as-synthesized PBA compounds is bound water. As explained in detail below, bound water can be reduced using temperatures greater than those required to remove the zeolitic water. However, it is unlikely that this type of water can be completely removed without degrading the PBA compound. Unlike zeolitic water, which can freely escape into a non-aqueous electrolyte, the chemical interaction between bound water and PBA restricts the presence of water in a non-aqueous electrolyte.
It would be advantageous if a PBA compound could be made using a process that completely removed zeolitic water and at least minimized the bound water content.
It would be advantageous if a battery or capacitor could be fabricated with a PBA cathode capable of efficiently working in voltage ranges greater than 2 V.    [1] V. D. Neff, Some performance characteristics of a Prussian Blue battery, Journal of Electrochemical Society, 132 (1985) 1382-1384.    [2] N. Imanishi, T. Morikawa, J. Kondo, Y. Takeda, O. Yamamoto, N. Kinugasa, T. Yamagishi, Lithium intercalation behavior into iron cyanide complex as positive electrode of lithium secondary battery, Journal of Power Sources, 79 (1999) 215-219.    [3] Y. Lu, L. Wang, J. Cheng, J. B. Goodenough, Prussian blue: a new framework for sodium batteries, Chemistry Communication, 48(2012)6544-6546.    [4] L. Wang, Y. Lu, J. Liu, M. Xu, J. Cheng, D. Zhang, J. B. Goodenough, A superior low-cost cathode for a Na-ion battery, Angew. Chem. Int. Ed., 52(2013)1964-1967.    [5] A. Eftekhari, Potassium secondary cell based on Prussian blue cathode, J. Power Sources, 126 (2004) 221-228.    [6] C. D. Wessells, R. A. Huggins, Y. Cui, Copper hexacyanoferrate battery electrodes with long cycle life and high power, Nature Communication, 2(2011) 550.    [7] C. D. Wessells, S. V. Peddada, R. A. Huggins, Y. Cui, Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium and potassium ion batteries, Nano Letter, 11(2011) 5421-5425.    [8] C. D. Wessells, S. V. Peddada, M. T. McDowell, R. A. Huggins, Y. Cui, The effect of insertion species on nanostructured open framework hexacyanoferrate battery electrode, J. Electrochem. Soc., 159(2012) A98-A103.    [9] J. Song, L. Wang, Y. Lu, J. Liu, B. Guo, P. Xiao, J.-J. Lee, X.-Q. Yang, G. Henkelman, J. B. Goodenough, “Removal of Interstitial H2O in Hexacyanometallates for a Superior Cathode of a Sodium-Ion Battery”, J. Am. Chem. Soc., 137(2015)2658-2664.    [10] L. Wang, J. Song, R. Qiao, L. A. Wray, M. A. Hossain, Y.-D. Chuang, W. Yang, Y. Lu, D. Evans, J.-J. Lee, S. Vail, X. Zhao, M. Nishijima, S. Kakimoto, J. B. Goodenough, “Rhombohedral Prussian White as Cathode for Rechargeable Sodium-Ion Batteries”, J. Am. Chem. Soc., 137(2015)2548-2554.    [11] X. Wu, W. Den, J. Qian, Y. Cao, X. Ai, H. Yang, Single-crystal FeFe(CN)6 nanoparticles: a high capacity and high rate cathode for Na-ion batteries, J. Mater. Chem. A., 1(2013)10130-10134.    [12] M. B. Robin, The color and electronic configurations of Prussian blue, Inorganic Chemistry, 1(1962)337-342.    [13] You, Y., Wu, X.-L., Yin, Y.-X. & Guo, Y.-G. High-quality Prussian blue crystals as superior cathode materials for room-temperature sodium-ion batteries. Energy & Environmental Science 7, 1643-1647.    [14] Colin Wessells et al., High Rate, Long Cycle Life Battery Electrode Materials with an Open Framework Structure, US 2012/0328936, published Dec. 27, 2012.    [15] Colin Wessells et al., Prussian Blue Analogue Anodes for Aqueous Electrolyte Batteries, US 2014/0220392, published Aug. 7, 2014.    [16] Colin Wessells et al., Cosolvent Electrolytes for Electrochemical Devices, US 2014/0308544, published Oct. 16, 2014.